US8050511B2 - High dynamic range images from low dynamic range images - Google Patents

Abstract

A method for displaying an image includes receiving an image having a first luminance dynamic range and modifying the image to a second luminance dynamic range free from being based upon other images, where the second dynamic range is greater than the first dynamic range. The modified image is displayed on a display.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Ser. No. 60/628,762 filed Nov. 16, 2004 entitled Using Spatial Assessment to Increase the Dynamic Range of Imagery and claims the benefit of U.S. Ser. No. 60/628,794 filed Nov. 16, 2004 entitled Generating High Dynamic Range Image Data From Low Dynamic Range Image Data by the use of Spatial Operators.

BACKGROUND OF THE INVENTION

The present application relates to increasing the dynamic range of images.

Many scenes existing in the real world inherently have extremely high dynamic range. For example, white paper in full sunlight has a luminance level of 30,000 cd/m^2, while white paper in the full moon has a luminance level of 0.01 cd/m^2, giving a dynamic range of 3 million to one (or 6.4 log units). The human eye can see even dimmer levels than 0.01 cd/m^2, so the visible range is even greater. In most situations, the dynamic range of a single scene is usually not this great, but it is frequently in excess of 5 log units. The human eye can only see 2-3 log units at a given instant, but is able to adjust the range via light adaptation, which can be less than a few seconds for the smaller adjustments, such as being able to go from reading a magazine in the sun to looking into the shadow under a car. More extreme range changes, such as going into a movie theatre from daylight, can take more than a minute.

Since traditional displays (both soft copy and hard copy) are not capable of displaying the full range luminances of the real world, a luminance mapping transfer is used to map from the dynamic range of the real world to the lower dynamic range of the display. Generally this mapping is performed in the image capture stage, and examples include the shoulder of D-Log-E curve for film, saturation for CCD sensors, or clipping in the A/D stages of such capture processes. These mapping functions are generally point processes, that is, ID functions of luminance that are applied per pixel (in the digital versions).

Computer graphics can generate images in floating point that match the luminances of the real world (generally, radiance approaches). In addition, some digital cameras similarly capture images with 12 to 16 bits per color. These are usually represented in a 16-bit format (examples: Radiance XYZ, OpenEXR, scRGB). But these digital images cannot be traditionally displayed without conversion to the lower dynamic range of the display. Generally the mapping algorithms for conversion from a greater to a lower dynamic range for the display capabilities are referred to as Tone Mapping Operators (TMO).

Tone Mapping Operators can be point processes, as mentioned for film and digital capture, but they can include spatial processes as well. Regardless of the type of TMO, all the approaches have traditionally been designed to go from a high dynamic range (HDR) image to a lower dynamic range (LDR) display (this term encompasses standard dynamic range, SDR).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a comparison of dynamic images for standard and high dynamic range displays.

FIG. 2 illustrates a standard dynamic range luminance versus high dynamic range luminance.

FIG. 3 illustrates a gamma adjusted standard dynamic range luminance versus high dynamic range luminance.

FIG. 4 illustrates standard dynamic range code value verses high dynamic range code values.

FIG. 5 illustrates a mapping of dynamic images for standard and high dynamic range displays.

FIG. 6 illustrates luminance profiles of diffuse and glossy curved surfaces.

FIG. 7 illustrates low dynamic range glossy surface luminance profile.

FIG. 8 illustrates low dynamic range image of glossy surface luminance profile using tone scale highlight compression.

FIG. 9 illustrates standard dynamic range code values verses high dynamic range code values with a modified mapping.

FIG. 10 illustrates low dynamic range image where diffuse maximum is clipped.

FIG. 11 illustrates low pass filtering to estimate specular highlight.

FIG. 12 illustrates a global technique for low dynamic range to high dynamic range mapping.

FIG. 13 illustrates another local technique for low dynamic range to high dynamic range mapping.

FIG. 14 illustrates a mapping of standard dynamic range code values to high dynamic range code values.

FIG. 15 illustrates linearly scaled low dynamic range image (top left), specular highlight candidate I (top right), specular highlight candidate2 (bottom left), and image re-scaled with piecewise linear technique (bottom right).

FIG. 16A illustrates a fixed range allocated to specular highlight region.

FIG. 16B illustrates a fixed slow allocated to diffuse image.

FIG. 17 illustrates adaptive slope parameters.

FIG. 18 illustrates an adaptive slope technique.

FIG. 19 illustrates tone scaling.

FIG. 20 illustrates a linearly scaled image (left) and a piece wise linearly scaled image (right).

FIG. 21A illustrates mixed layer clipping of specular highlights.

FIG. 21B illustrates a technique for using color ratios if one of the colors is not clipped.

FIG. 22 illustrates range allocation using standard dynamic range white.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Newly developed displays have been made which have substantially higher dynamic range than the traditional state of the art displays. The general difference in dynamic ranges for the newly developeddisplays110 and thetraditional displays120 is shown inFIG. 1 for a log luminance scale. Some current state of the art standard dynamic range displays may have a range of 500 cd/m^2 to 0.7 cd/m^2. The newly developed “high dynamic range” displays may have a range from 3000 cd/m^2 to 0.05 cd/m^2, or even lower. In existing display technologies the image data is displayed on the display with its existing dynamic range.

The present inventors came to the realization that the image being presented on the display could be subjectively improved if the dynamic range of the image data is effectively increased. Since most images are already represented in a LDR (low dynamic range) format, a technique is desirable to convert the image from LDR up to HDR (high dynamic range).

One technique suitable to perform a mapping from a lower dynamic range image to a higher dynamic range image suitable for display on a higherdynamic range130 display is shown inFIG. 2. The technique includes a linear stretch from the lower dynamic range shown on the horizontal axis, to the higherdynamic range140, shown on the vertical axis. The horizontal axis is shown as shorter than the vertical axis to convey the smaller range. On the left, the axes are in terms of actual luminances.

The technique illustrated inFIG. 2 tends to result in a somewhat ‘flat’ contrast in the modified image. To improve the contrast, referring toFIG. 3, anonlinear mapping150 using a gamma function, or another suitable function, is used to increase the contrast. The axes are shown in units of luminance.

The technique illustrated inFIG. 4 shows a linear stretch where the axes are in code values. Since the code values are generally nonlinear in luminance, this is equivalent to a nonlinear mapping, such as is shown inFIG. 3. In the illustrations shown inFIGS. 2,3, and4, the TMOs may be non-adaptive “point processing” approaches. They do not use spatial processes, nor do they change depending on the contents of the image. It is to be understood that the processes may be spatial processes and change depending on the content of the image, if desired.

For HDR displays that have high dynamic range at the pixel resolution, the linear stretch technique increases the amplitude gray level resolution (i.e., more actual bits, rather than just adding 0s or 1s to the LSBs, which typically occurs in the linear scaling approach). For other HDR displays, such as multiband versions, where two modulating layers are used that have differing resolution, the increase in actual bits is not necessary, if desired.

In many cases, the black point and the white point for different image sources are different. In addition, the black point and the white point for different displays are different. It is desirable to adjust the mapping for a lowerdynamic range160,165 to a higher dynamic range170 (e.g., luminance and/or digital values) to account for the different image sources and/or the different displays. Referring toFIG. 5, an illustration shows a grey level range for an exemplary highdynamic range170, as well as the endpoints (light end and dark end) mappings that may be used, as expressed in luminances, for different types ofstandard160,165 (i.e. lower) dynamic ranges. Further, these SDR ranges may result from the storage format of the input digital image (e.g., 8 bits/color sRGB format), or from the image capture process (film has around 5-10 stops or a 1.7-3 log unit range, radiographs can be as high as 4 log units). However, for display purposes they are usually digitized and represented with less than 2 log units of range.

The preferred embodiment may use the properties of the physical specular highlights, as well as the typical ways in which the specular highlight is modified by the image capture (or representation format), to re-create the physical specular highlight properties on the display.FIG. 6 shows anexemplary luminance profile180 of a diffuse surface. The physical surface may be curved or flat, as both may lead to this general shape. A flat surface will lead to this general shape if it is Lambertian (a type of diffuse) and the light source is not at infinity. A curved surface will likewise lead to this type of general profile even if the light source is at infinity.FIG. 6 also shows an exemplaryphysical luminance profile190 if the surface is glossy. The narrow region withhigh luminance200 is referred to as the specular highlight, and it primarily has the color of the light source, as opposed to the color of the diffuse object. In many cases there is some mixing of the object color and the light source color.

The amplitude of the specular highlight is very high in the physical world. It can be as high as 100 times the luminance of the diffuse reflected luminance. This can occur even if the diffuse object is white. This large amplitude specular highlight is not captured in the low dynamic range image. If it was, then since the LDR image range is usually less than 2.5 log units and since the specular range can be as high as 2 log units, most of the image will be concentrated in the lower 0.5 log units, and will be nearly black. So in a LDR image, the specular highlight is reduced in amplitude in several ways.

One of the ways the specular highlight is reduced is by clipping. Values out of range are simply set to themaximum value210, as shown inFIG. 7. This occurs in a good exposure, where care was taken not to set the image maximum to the diffuse maximum. That is, some shape of the specular highlight will be visible in the image (generally correct in position and shape), but the specular highlight won't be as bright relative to the object as it is in the physical world. The consequence is that the image looks less dazzling, and less realistic.

Referring toFIG. 8, another way the specular highlight can be reduced is via tonescale compression. An example is that resulting from the s-shaped tone response curve of film. Here the position, spatial shape, and even relative luminance profile of thespecular highlight220 is preserved, but the actual amplitude is reduced (as in the clipped case).

As illustrated, the preferred technique includes a linear scaling of the LDR image to the HDR image. The scaling may likewise include a decontouring technique to generate additional bit depth. Other techniques may likewise be used to generate additional bit depth. In addition non-linear scaling may be used.

The characterization of the specular highlight, as previously discussed, may be used at least in part to expand the tone scale of the image. Referring toFIG. 9, theimage region230 where the luminance falls below the diffuse maximum240 is tone mapped with a lower slope than theimage regions250 that have captured the specular highlight. The specular highlightmaximum value260, as found in the image, Specular_max, as well as the diffuse regionmaximum value240, Diffuse_max, appear on the horizontal axes. This axis corresponds to the input image digital code values. It is to be understood that these values may likewise be approximate. In general, the range from zero to Diffuse_max(or another appropriate value) has a greater amount of the code value range or otherwise a greater range of luminance than the range from Diffuse_max,to Specular_max. As a general matter, the lower range of luminance values of the input image are mapped with a first function to the luminance values of the high dynamic range image, and the upper range of luminance values of the input image are mapped with a second function to the luminance values of the high dynamic range image, where the first function results in a denser mapping than the second function. One way to characterize the denser mapping is that the first function maps the diffuse to lower values with a lower tonescale slope than the specular image for the second function.

The two gray levels extracted from the LDR input image are mapped to the HDR display to its luminances, as shown. The Specular_maxis mapped to the HDR displays'maximum value270, while the Diffuse_maxis mapped to a value referred to asD_HDR280, referring to the diffuse max point as displayed on the HDR display. One can see a certain amount of the dynamic range of the HDR display is allocated for the diffusely reflecting regions, and a certain amount is allocated to the specular. The parameter D_HDRdetermines this allocation. Allocating more to the specular highlight makes the highlights more dazzling, but results in a darker overall image. The decision is affected by the actual range of the HDR display. For very bright and high ranging HDR displays, more of the range can be allocated to the specular region without having the image appear dark.

In some images with a poor exposure, even the diffusemaximum value290 is clipped, as shown inFIG. 10. In these cases there is a complete loss of any specular highlight info. That is, the position, the shape, and the luminance profile of the highlight is substantially missing. In those cases the system may selectively determine that there is no need to attempt to restore the highlight.

In order to most effectively use the tonescale ofFIG. 9, the systems determines the Specular_maxand Diffuse_maxvalues from the image. This may be done by first finding the maximum of the image, and assume it is the specular maximum. This is generally not the case if there is a large amount of noise or if the image contains no specular highlights.

The system also determines the diffuse maximum from the image. The technique involves removal or otherwise attenuate the specular highlight. In general the specular highlight has anticipated characteristics, such as it may be a small isolated region, it may be relatively narrow with high amplitude in the physical scene, but in the LDR image, it tends to be narrow with small amplitude. The system may use a low-pass filter300 to reduce thespecular highlight310, as shown inFIG. 11. For example, the low pass filter may be large enough so that the result is too blurry320 to be used for actual viewing. That is, the LPF step is used to identify the diffuse maximum of the diffuse image.

For the case where even the diffuse maximum has been clipped (seeFIG. 10), then the image maximum and the LPF image maximum will be substantially the same. This is also true in cases where there is no significant specular highlight. The maximum found is then assumed to be the diffuse maximum. In both cases, then the tone mapper does not place any image regions in the region with increased slope for specular highlights. It can then use the tone mapper fromFIG. 9 (where the found image max is set as the diffuse max) or the general linear stretch fromFIG. 4 where the found image max sets the image max.

FIG. 12 illustrates another technique for performing this image modification. Theinput SDR image400 is used estimate the diffusemax402 andspecular max404 usinglow pass filtering406 andmaximum operators408. These parameters are input to aprocess410 for determining a tone mapping operator (TMO). The TMO fromprocess410 is applied to theinput image400 to provide an image for theHDR display414.

In many cases existing high dynamic range data formats are in the linear space and high dynamic range displays are designed to operate in the linear space. In contrast, many low dynamic range data formats are represented in the gamma domain (e.g., sRGB). While either the linear space (substantially linear) or the gamma space (substantially non-linear) may be used for image processing, it is preferable to use the linear space because the understanding of the physics of specular highlights is more understood in the linear space. If the input image is not in the preferred format, such as linear domain or gamma domain, then the input image may be converted to the preferred domain.

While the system functions well, it turns out that the techniques sometimes do not detect some of the specular highlights. After further examination it was determined that some specular highlights are difficult to detect because the specular highlights are not always saturated (e.g., clipped), the specular highlights can be in 1, 2, and/or 3 of the color channels (e.g., in the case of three color channels), the size of the specular highlights is usually small in a scene and varies on how the picture was obtained, and that the specular highlights are often of a regular shape but not always circular in nature primarily due to the projection of the image on the image plane.

It has been determined that since the specular highlights are not always saturated, a fixed threshold may have a tendency to miss specular highlights. Based upon this observation, the system preferably uses an adaptive threshold. The preferred technique computes a low-pass filtered image and assumes that it corresponds to the diffuse image.

Initially the specular image specI is defined as follows:
Ti=max(lowpass(I))
specI=I>T1

The size of the low-pass filter is preferably based on the assumption that specular highlights are small and bright. An example includes 11 taps for an image of size 1024 vertical (e.g., XGA), and is scaled accordingly for different image sizes. Additional morphological operations may then be used in order to include the pixels that were rejected by the threshold but are likely to be part of the specular highlights. An exemplary technique is shown inFIG. 13.

Specular highlights tend to be very bright and they can be as bright as 2 log units or more over the diffuse maximum value. Even allocating 1 log unit to the specular highlights mean that about 1/10^thof the dynamic range should be allocated to diffuse image while 9/10^thshould be allocated to specular highlights. That is not generally feasible, with the exception of very bright HDR displays. Accordingly, achieving the actual max possible dynamic range of 2 logs for specular highlights may not be desirable in many cases.

Based upon an understanding that the range allocated to the specular highlights will be less than that of the physical world, a study was conducted to estimate what range should be allocated to specular highlights using images that were segmented by hand so that an assumption could be made based upon ideal specular highlight detection. With these isolated specular highlights, two primary different methods for scaling were investigated, namely, ratio scaling and piecewise linear scaling.

With respect to ratio scaling, the motivation for this technique is that in some cases, not all three color channels have clipped specular highlights (i.e., image's specular highlights are a mixture ofFIGS. 7 and 8 across the color bands). Since the specular highlight generally reflects the color of the light source, this situation occurs with non-white light sources. The principle is to look for the ratio or other relationship between color channels in the region just outside where clipping has occurred, and deduce the value of the clipped channels by generally maintaining the RGB ratios relative to the unclipped channel. However, in the case that all three color channels are saturated (clipped) this technique not especially suitable. Moreover, it has been determined that the ratio can differ drastically from one pixel to the other along the specular highlight contour. Further, even if one of the specular highlights is not clipped, there is a good chance it is has been compressed via an s-shaped tonescale (seeFIG. 8).

With respect to piecewise linear scaling, this technique scales the image with a two slope function whose slopes are determined by the maximum diffuse white. The function parameters are the maximum diffuse white, specular highlight max (usually the image max) and the range allocated to the specular highlights. It is possible to use a fixed slope and/or a fixed range. However, to reduce visible artifacts between diffuse and specular parts of an image, it is preferable to use an adaptive function that changes the allocated range from ¾ to ⅓ depending on the maximum diffuse white.

Referring toFIG. 13, theimage500 processing include the following:

1) Low pass filter withfilter F1502;

• • a.detemiine maximum506 of the low passedimage502;
  • b. use maximum506 to determinethreshold T1508;
  • c. use thethreshold T1508 to modify theimage500 with athreshold operation504;

The process preferably uses a low-pass filter that is about 1/100^thof the image dimension (11 pixels for a 1024 image) based upon the luminance.

2) Low pass filter withlowpass filter F2510, (F2>F1, spatially)

• • a. determine maximum512 of thelow pass image510;
  • b. use maximum512 to determinethreshold T2514;
  • c. use thethreshold T2514 to modify theimage500 with adilation operation516;

The process preferably uses a low-pass filter that is about 1/50^thof the image dimension (21 pixels for a 1024 image) based upon the luminance.

3) Thethreshold operation504 of theimage500 withT1514 determines the 1^stspecular highlights candidates520, which may be in the form of a binary map, if desired.

4) Refinebinary map520 with an erosionmorphological operator522 to provideSH candidate2524.

• • a. theerosion522 removes single pixels (parameter set as such) and also reducesfalse SH candidates520 due to noise that was clipped.

5) Refinebinary map524 with the dilationmorphological operator516 to provideSH candidate3530;

• • a. thedilation516 is constrained byT2514;
  • b. If pixel>T2 and 4 neighbors are specular highlight candidates->pixel=SH candidate
  • c. threshold T2514 serves as a constraint to limit the expansion.

6)Mask540 theinput image500 with thespecular highlight map530;

• • a. i.e. if pixel not SH, then ignore by masking out the pixel value to provide amasked image542.

7) Find maximum diffuse white (MDW)544 by taking theminimum546 of themasked image542;

• • a. this provides the minimum of image in specular highlight region;
  • b. due to the constrained morphological operator, it is likely that the maximum of the diffuse image be larger than the minimum specular images. This reduces the bright areas of the diffuse image to be boosted up as well.

8) Generate tonescale (tone mapping operator, TMO)550 usingMDW544 and range desired for specular highlight;

• • a. an adaptive slope technique is preferred.

9) Process theinput image500 based upon theTMO550 by applying thetone mapping552;

• • a. one approach is to run entire image through the single TMO;
  • b. other approach is to use different TMOs for each class of pixel (specular highlight and non-SH) using the binary map.

10)Output image560 is sent to the HDR display.

This technique may presume that the specular highlights are small and bright for improved effectiveness. That means that large bright light source such as sun will not likely be detected by this technique.

InFIG. 9, the tone-mapping operator has a sharp transition between the two regions. In practice it is better to have asmooth transition600 between the two regions, such as illustrated inFIG. 14.

InFIG. 15, results are shown comparing the technique ofFIG. 13 to linear scaling. The results are best viewed on a HDR display. Nevertheless, one can observe the difference in the images and how the specular highlights are much brighter than their underlying diffuse regions.

Parameters may be set to determine what dynamic range should be allocated to the specular highlights. Because the specular highlights are often very bright (2 log units) and the detection part determines the maximum diffuse white (MDW) making it image dependent, the scaling function may include an additional slope based parameter (fixed slope/fixed range). As a result the system may include an adaptive slope technique.

The prevent inventors considered optimization originally to include determining a range to be allocated to the specular highlights. However, it was determined that the scaling function could significantly change with different maximum diffuse white (MDW) resulting from the detection functionality.FIG. 16A shows two examples of scaling functions with variable MDW.FIG. 16A shows the scaling functions obtained by varying MDW while keeping the range allocated to the specular image (R) constant.FIG. 16B shows the scaling functions obtained by varying MDW while keeping the lower slope constant. It was determined that using an adaptive slope technique reduces the variability of the scaling functions.

With the adaptive slope technique, the allocated range depends on the MDW value as illustrated inFIG. 17. The motivation is to have less variability with different mdw values:

1. the allocated range depends on mdw value.

2. the binary map is used for the scaling;

• • a. The specular image is computed with the steepest part of the function;
  • b. The diffuse image is scaled with the first slope (even if the diffuse white is brighter than the maximum diffuse white).

3. With the adaptive slope method, the allocated range depends on mdw value;
slope=(R_max−R_min)/(Mdw_max−Mdw_min);
R=slope−Mdw−(Mdw_min×slope−R_min).

The preferred embodiment values, as illustrated inFIG. 17, are
R_max=170, R_min=64, Mdw_max=230, Mdw_min=130.

FIG. 18 illustrates an exemplary adaptive slope set of curves.

Since the maximum of the diffuse part is larger than the minimum of the specular image, the specular candidate binary map should be taken into account during the tone scaling operation. The scaling takes into account spatial information, as illustrated inFIG. 19 where theline510 shows the scaling function that is applied on the diffuse image and theline512 shows the scaling function that is applied to the specular image. This is the two TMO approach mentioned previously.

One technique to assess the quality of the processed images is to compare it with an image that was scaled using a simple linear method.FIG. 20 compares a linearly scaled image with an image scaled by piece-wise linear technique. Note how the specular highlights look brighter on the right image. Of course these two images should be compared on a high dynamic range display for most effectiveness.

Some alternatives and modifications include:

• • 1. Selection of D_HDR, the luminance of the image diffuse maximum as displayed on the HDR display
  • 2. The width of the transition region.
  • 3. The size and shape of the low-pass filter F1, which can affect Diffuse_max.
  • 4. The size and shape of the low-pass filter F2, which can affect Diffuse_max.
  • 5. Use of a noise-removinglow pass 3×3 filter is already applied.
  • 6. Whether nonlinearities, such as gamma correction, are used on the two-tone mapping regions for diffuse and specular highlights.

Referring toFIGS. 21A and 21B, a method that uses the color ratio method to predict the clipped specular highlights is illustrated. The image profile of partially clipped specular highlights is shown inFIG. 21A and the technique for reconstruction of these is shown inFIG. 21B. Note that this technique can be used as a preprocessor for the principal technique (FIG. 13), for both outcomes.

Another technique uses the HDR display so that its white point matches SDR displays (i.e., a given benchmark SDR display), and then the rest of the brightness capability of the HDR is allocated to the specular highlights. This approach works with “real” values instead of the ratio for the scaling. Instead of using the adaptive slope method to compute the range allocated to the specular highlight, the system could use the white point of standard LCDs display to define the range R. Then, all values brighter values brighter than R are specular highlights and will only be visible when displayed on the HDR monitor. This is illustrated inFIG. 22.

Another technique does not use the nonlinear TMO. This idea is based on the fact that if an image is scaled linearly from 8 to 16 bits, the contouring artifacts typically appear. In that case, the decontouring algorithm can be used to provide good HDR image from one LDR image. However, due to the properties of some HDR monitors such as (low-resolution of the led layer), no contour artifacts appear even after a linear scaling. Plus, this technique does not generate more realistic specular highlights, but it does extend the dynamic range and provides a linear scaling free of contours artifacts. The technique may adapt the coring function to 16 bits images and compare linearly scaled images against linearly scaled images after decontouring.

The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims (10)

1. A method for displaying a color image comprising:

(a) receiving an image having a first dynamic range;

(b) modifying said image with a computing device to a second dynamic range, wherein said second dynamic range is greater than said first dynamic range;

wherein said modification includes the steps of:

(i) converting a received said image which is in a substantially gamma domain to a substantially linear luminance domain;

(ii) processing at least two color channels of said image, in said substantially linear luminance domain to assign a first luminance value as a maximum diffuse luminance of said image;

(iii) said modifying is based upon a scaling operation that scales luminance values using a pair of linear functions each linear in a luminance domain of said image and joined together at a second luminance value of said substantially linear luminance domain in the respectively processed said at least two color channels, where said second luminance value is less than said first luminance value by an amount calculated as a function of said first luminance value; and

(c) displaying said modified image on a display.

2. The method ofclaim 1 wherein the lower range of said first dynamic range is mapped to the lower range of said second dynamic range with a first function, wherein the higher range of said first dynamic range is mapped to the higher range of said second dynamic range with a second function, wherein said first function has a denser mapping than said second function.

3. The method ofclaim 2 wherein said first dynamic range is a first luminance dynamic range, and said second dynamic range is a second luminance dynamic range.

4. The method ofclaim 1 wherein said at least two color channels are compared to one another.

5. The method ofclaim 4 wherein said comparing occurs proximate a region of clipped highlights.

6. The method ofclaim 1 wherein said modifying is based upon said function without discontinuities.

7. The method ofclaim 1 wherein said first dynamic range is a first luminance dynamic range, and said second dynamic range is a second luminance dynamic range.

8. The method ofclaim 6 wherein said first dynamic range is a first luminance dynamic range, and said second dynamic range is a second luminance dynamic range.

9. A method for displaying an image having specular highlights, said method comprising:

(a) receiving an image having a first dynamic range;

(b) modifying said image with a processing device to a second dynamic range using a transformation that allocates a specular portion of said second dynamic range to display said specular highlights and a diffuse portion of said second dynamic range to display diffuse tones of said image, wherein said second dynamic range is greater than said first dynamic range, and wherein said specular portion of said second dynamic range has a size calculated as a function of a maximum diffuse luminance value received from said image;

(c) displaying said modified image a display.

10. The method ofclaim 9 wherein said transformation uses a plurality of piecewise functions where the slope of at least one said piecewise function is adaptively determined.

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